Message queue solution for activities that come and go on Android?

Lets say I have a Service S and Activity A. S downloads data for A (or handles some long running work, whatever), but A is not always present. I don't want S to hang around when it's job queue is empty: S should post the results of the finished works to some kind of a mailbox for A, so A can pull the messages when it comes back again.

Can this be achieved without using SQLite of file s开发者_高级运维torage for the implementation of the mailbox? I'd prefer some faster mechanism, write operations tend to be quite slow on a device. I thought about using a simple static list inside the ApplicationContext, but afaik relying on the ApplicationContext results a risky/fragile solution.

Could anyone recommend a pattern for this problem?

Can this be achieved without using SQLite of file storage for the implementation of the mailbox?

Not reliably. Either it's a file, or it might be nuked before A comes back again. Remember that your process -- where all your static data members and the Application object reside -- does not live forever. Once S shuts down (which is a good thing, thanks!), Android is welcome to terminate the process, taking your "mailbox" with you if it is solely in RAM.

You could persist it to disk yet keep a singleton or something around as a cache, so if A returns quickly you can skip some of the I/O. Or, if it does not really matter much if the messages exist for A, you could keep them in RAM and simply shrug your shoulders if the process gets terminated first.